Recently, a frequency used for communication is expanding to a high-frequency region of microwave region or milliwave region along with a rapid development of communication networks. As a dielectric ceramic composition used to produce an electronic component for such a high-frequency circuit (electronic component for high-frequency use), there is required a material wherein the loss coefficient Qm (may be also referred to as Q simply) value is large and further the absolute value of the temperature coefficient τf of resonance frequency fo is small and can be easily adjusted to a desired value.
As the value of the relative permittivity ∈r of a dielectric ceramic composition for high-frequency use becomes larger, the size of an electronic component for high-frequency use constituting a high-frequency circuit such as a microwave circuit, a milliwave circuit or the like can be reduced. However, when the relative permittivity ∈r of the dielectric ceramic composition for high-frequency use used for the electronic component for high-frequency use is too large in high-frequency regions of microwave and milliwave, the electronic component for high-frequency use is too small in the size and the processing accuracy is severe and hence the productivity is deteriorated. Thus, the relative permittivity ∈r of the dielectric ceramic composition for high-frequency use is required to be an appropriate size. The electronic component for high-frequency use varies size depending on a frequency to be used. It is, therefore, demanded that a material for an electronic component for high-frequency use can easily obtain (that is, adjust) a required relative permittivity ∈r in order to achieve a high-frequency electronic component for high-frequency circuits such as microwave circuit, milliwave circuit and the like with both features of improvement of processability and miniaturization.
Heretofore, as a dielectric ceramic composition for high-frequency use, there are proposed BaO—MgO—WO3-based materials (see Patent Document 1), MgTiO3—CaTiO3-based materials (see Patent Document 2) and so on. However, since each of these dielectric ceramic compositions for high-frequency use has a relative permittivity ∈r of not less than 13, as the frequency used becomes higher, there is required a dielectric ceramic composition for high-frequency use having a less relative permittivity ∈r. Moreover, there is a problem in these dielectric ceramic compositions for high-frequency use that the relative permittivity ∈r can be adjusted only within a relatively narrow range in a composition region showing an absolute value of a temperature coefficient τf of resonance frequency of near 0 ppm/° C.
On the other hand, alumina (Al2O3), forsterite (Mg2SiO4), cordierite (Mg2Al4Si5O18) and so on are excellent in the Qm value, so that they can be used for a electronic circuit substrate and so on. However, since the temperature coefficient τf of resonance frequency is −30 to −70 ppm/° C., their use is limited. When these materials incorporate impurities, there is a problem of having a major influence on a formation phase and electric properties, or the like.
There is further proposed a ceramic composition composed of forsterite (Mg2SiO4), calcium titanate (CaTiO3) and spinel (see Patent Document 3). However, although Patent Document 3 discloses that the temperature dependency of the relative permittivity ∈r of the ceramic composition can be controlled, there is not disclosed at all a value of relative permittivity ∈r, or a possibility to control or adjust it.
There is also proposed a dielectric ceramic composition wherein titanium oxide (TiO2) is added to forsterite (Mg2SiO4) (see Non-Patent Document 1). In the dielectric ceramic composition, the temperature coefficient τf of resonance frequency is gradually shifted to a plus side along with the adding of titanium oxide (TiO2). However, the temperature coefficient τf of resonance frequency is a large negative value of −62 ppm/° C., even if 30 wt % of titanium oxide is added. Thus, it is impracticable.
By the way, the most basic dielectric resonator includes a coaxial dielectric resonator. In the coaxial dielectric resonator, a block composed of a dielectric ceramic is provided with a through-hole, and only one surface of the block in which the through-hole opens (opened surface) remains the surface of the dielectric ceramic, and a conductive film is formed on other surfaces of the dielectric ceramic and the inner surface of the through-hole.
The most basic waveguide as a high-frequency planar circuit element includes a microstrip line. In the microstrip line, one surface of front and back surfaces of a dielectric ceramic substrate is provided with a strip conductor, and another surface of the dielectric ceramic substrate is provided with a ground conductive film.
The above coaxial dielectric resonator and microstrip line can be used to constitute a microwave transmitter of a dielectric resonator control type. In the microwave transmitter, a coaxial dielectric resonator is mounted on a dielectric ceramic substrate through a supporting member composed of a dielectric ceramic, and the coaxial dielectric resonator is coupled to a microstrip line provided on the dielectric ceramic substrate with an electromagnetical field which is leaked outside the coaxial dielectric resonator.
In this kind of high-frequency circuit, a resonance system having a high non-loaded Q is constituted by suppressing leakage of electrical field through a supporting member. Thus, it is required to use a material having a low relative permittivity and a small dielectric loss (tan δ) (that is, a large Qm×fo) as a material for a supporting member. Heretofore, as the material for a supporting member, there is used forsterite (Mg2SiO4) wherein the relative permittivity ∈r is about 7 and Qm×fo is about 150000 GHz. As a material for a dielectric ceramic substrate, there are mainly used alumina ceramics (Al2O3) wherein the relative permittivity ∈r is about 10 and Qm×fo is not less than 200000 GHz (for example, see Patent Document 4). However, the temperature coefficient τf of resonance frequency of these materials is prone to take −30 to −70 ppm/° C. and hence the use of the high-frequency circuit is limited. When these materials incorporate impurities, there is a problem that the constitution of the formation phase and the electric properties vary largely, or the like.
Also, a dielectric ceramic based on the dielectric ceramic composition described in Non-Patent Document 1 is impracticable.
On the other hand, as a material for a dielectric ceramic substrate constituting a dielectric waveguide, there are generally used teflon (registered trademark) and alumina ceramics (Al2O3). However, the temperature coefficient τf of resonance frequency of these materials is prone to take −30 to −70 ppm/° C. and hence the use of the high-frequency circuit is limited.
There is a development example of applying a dielectric material wherein relative permittivity ∈r=24, Qm×fo=350000 GHz and temperature coefficient τf of resonance frequency=0 ppm/° C. to a planar filter (Non-Patent Document 2), but it is required that a relative permittivity ∈r is not more than about 12, Qm×fo is not less than 40000 GHz, preferably not less than 50000 GHz and an absolute value of a temperature coefficient τf of resonance frequency fo is not more than 30 ppm/° C. in order to respond to the request to heighten further a frequency in the future.
As a frequency region becomes higher, the impact of skin effect becomes larger. For example, when Ag is used as a conductive material, a skin depth at a region of 1-3 GHz is 1.18-2.04 μm (Non-Patent Document 3).